1. Field of the Invention
This invention is related to managing trunk traffic in a telecommunication switch. In particular, this invention involves performing circular hunts for an idle trunk member. With this invention, circular hunts are performed in such away as to reduce the amount of search time and to more evenly distribute traffic across large numbers of trunk groups. The invention supports very large trunk groups that might otherwise encounter trunk timeout, or other conditions before a final disposition of a call is determined. The invention enforces a maximum number of trunk groups or routes searched on a per call basis such that trunk timeout or other anomalies are avoided by returning an all trunks busy condition if no idle members are found within the number of trunks or routes searched.
2. Description of the Problem Solved
The public switched telephone network (PSTN) consists of many switches interconnected by trunks. The trunks which interconnect the switches of the PSTN include multiple trunk groups, each trunk group having many members which can carry calls. Which trunk group is available to a specific call determines the route that the call will take to get to its destination. When a switch handles a call, it must search for an available route, that its, a trunk group with an available channel or member. One type of search used to find a route is called a "circular hunt," because it can be depicted graphically as a pointer moving around a circle on which the various trunk groups, or available routes, are shown around the circumference of the circle, as shown in FIG. 1, where the pointer indicates the current route. When a large number of trunk groups, routes, or channels is involved, a search of this or any type places a burden on the processing resources of the switch which results in a delay until the call can be completed.
With greatly increasing demand being placed on the PSTN, larger and larger numbers of trunk groups are connected to the various switches which make up the PSTN. Much of this demand is due to the growth of the Internet, and many of the trunk groups are connected to Internet access devices. The number of trunk groups can be so large that internal switch limitations can cause software errors and/or trunk timeouts if a free channel or member of a trunk group is not found quickly. Additionally, the usage of the various trunk groups is not evenly distributed, resulting in wasted resources. This problem is illustrated in FIG. 2. A trunk group is also called a route (RTE). FIG. 2 is a table which illustrates the problem in more detail for an ascending sequential search algorithm. A star indicates a particular channel or member of a particular trunk is in use. In this case the trunks are integrated services digital network (ISDN) primary rate interfaces (PRI). Note the upper channels of the higher numbered trunks are not being used at all. The diamond over trunk group PRI 1/channel 1 indicates where the search algorithm always starts in an ascending sequential search algorithm of the prior art. Traffic must fill all channels in PRI 1 before the algorithm searches PRI 2. PRI 2 must be completely filled before PRI 3 is used. The result is that trunk groups placed later in a route list are used much less frequently than trunk groups that are early in the route list. The DS-1 column indicates that a trunk group may have multiple DS-1's, or digital signalling-1 trunks, in the trunk group and is not limited to 23 bearer channels. For this illustration PRI with 23 channels plus one D (signaling channel) is used; as well as the scenario where for PRI 5 four DS-1's are used and two D (signaling) channels are used. One D channel is active and one is on standby (or backup). What is needed is a way to limit the number of trunk groups searched, while still insuring even distribution of traffic.